This invention related to apparatus and method for identifying multiple fluids in a column and determining the level of their interface, and more particularly to apparatus and method for using complex impedances measured between spaced electrodes in the liquids for making the aforementioned identification.
Storage tanks, particularly in the petroleum industry, are often relatively inaccessible due to size or underground location. These tanks often contain a plurality of different types of fluid, such as different types of oil, separated into layers by differences in density. Operators of storage facilities must have the ability to identify the specific fluids in these tanks and the location of the interfaces between different fluids in order to manage their resources.
The oldest known identification technique has been sampling; i.e., the use of sampling ports along the side of the tank at different levels, or the use of a container lowered into the tank and allowed to fill with the fluid at the level under examination. Sampling is a labor-intensive, slow, process requiring the judgment of a person to identify a product by sensual observation of its characteristics.
There is inherent difficulty in using sensual observation to distinguish among different types of oils because these substances appear quite similar. Accordingly, the traditional method of identification has been by chemical analysis of samples. The use of electrical properties to identify oils in situ is not known to the petroleum industry.
Electrical properties of fluids have routinely been used to determine the level of a surface of a fluid. An RF capacitance unit was developed in the 1930's to identify the level of fluids that relied on the capacitance charge induced by the presence of a fluid between two electrodes or between an electrode and a metal tank wall. This system required careful calibration and maintenance, and the accuracy of the instrument was highly dependent on environmental parameters. Many subsequent patents have been issued to level detection schemes using changes in capacitance or other electrical parameters.
The use of complex impedance to determine the interface between two fluids is discussed by U.S. Pat. No. 4,503,383 of J. Agar et al. This patent has a sealed three foot long probe to reduce the capacitance of the probe. According to Agar's patent, the particular probe structure enables the detector to be arranged to respond to the impedance of the fluids, the impedance being primarily resistive rather than capacitive. A change in this impedance is indicative of an interface between oil and brine.
A system of identifying fluids using thermal properties is shown in U.S. Pat. No. 4,575,260 of A. Young. This patent uses a multi-leg bridge circuit to provide for heating of the fluid and for compensation for ambient temperature. The patent does not teach the use of electrical characteristics for identification of fluids.